The paper presents the analysis of the performance of the InAs/InAsSb superlattice barrier detector operated at 230 K and long-wavelengths infrared spectrum (LWIR). To determine the position of the electron miniband and the first heavy hole state in the superlattice, we have used a k·p. Having the position of the conduction band and valence band we have to determine a correct band alignment between the barrier and absorber layer – the barrier in the valence band must be sufficiently low to ensure the flow of optically generated holes. We have considered an AlSb material for barrier best aligned to LWIR InAs/InAsSb superlattice absorber.
This article presents the InAs/GaSb type-II superlattice interband cascade photodetector low-frequency noise analysis. Presented noise measurements were conducted in temperature range 77-300 K and in frequency range 1-10 kHz. The spectral characteristics of the detector noise at various voltages are analyzed. The measured power spectral density is compared to the theoretical prediction. This comparison suggests that noise estimates based on the dark current may not always be valid and the noise source is difficult to be extract. We showed two samples, one of which was characterized by a large leakage currents contribution being directly translated into increased noise.
InAsSb/AlSb barrier detectors were grown on (100) semi-insulating GaAs substrates by a molecular beam epitaxy. We compare the performance of two detectors with different active layers denoted as p + BppBpN + and p + Bpnn + . InAs0.81Sb0.19 absorber allows to operate up to 5.3-μm cut-off wavelengths at 230 K. p + Bpnn + detector (n-type absorber) exhibits diffusion-limited dark currents above 200 K. AlSb barrier provides low dark currents and suppresses surface leakage currents. With a value of 0.13 A / cm2 at 230 K, the current is of about an order of magnitude larger than determined by the “Rule 07.” Dark currents of p + BppBpN + detector (p-type absorber) are much higher due to a contribution of Shockley–Read–Hall mechanisms. On the other hand, a device with a p-type absorber exhibits the highest value of current responsivity, up to 2.5 A / W, pointing out that there is a tradeoff between dark current performance and quantum efficiency.
An InAsSb/AlSb heterostructure photovoltaic detector structures were grown on a (100) semi-insulating GaAs substrates by a molecular beam epitaxy. We compare the performance of two detectors with a different type of absorbing layers, denoted p+BppBpn+ and p+Bpnn+. InAs0.81Sb0.19 absorption layers allow for a operation up to 5.3 μm cut-off wavelengths at 230 K. p+Bpnn+ detector (n-type absorber) exhibits diffusion-limited dark currents above 200 K. AlSb barrier provides a low values of dark currents and allows a suppression of surface leakage current. With a value of 0.13 A/cm2 at 230 K, the current is less than an order of magnitude larger than those determined by the "Rule 07" for HgCdTe detectors. Dark currents of p+BppBpN+ detector (p-type absorber) are much higher due to a contribution of Shockley-Read-Hall mechanisms. On the other hand, device with a p-type absorber shows highest values of current responsivity, up to 2.5 A/W, point out that there is a trade-off between dark current performance and quantum efficiency.
We investigate the high-operating temperature performance of InAsSb/AlSb heterostructure detectors with cutoff wavelengths near 5 μm at 230 K. The devices have been fabricated with different types of absorbing layers: nominally undoped absorber (with n-type conductivity), and both n- and p-type doped. The results show that the device performance strongly depends on absorber layer type. Generally, the p-type absorber provides higher values of current responsivity than the n-type absorber, but at the same time also higher values of dark current. The device with the nominally undoped absorbing layer shows moderate values of both current responsivity and dark current. Resulting detectivities D * of nonimmersed devices vary from 2 × 109 to 5 × 109 cm Hz1/2 W ? 1 at 230 K, which is easily achievable with a two-stage thermoelectric cooler. Optical immersion increases the detectivity up to 5 × 1010 cm Hz1/2 W ? 1.
The paper presents the performance of the interband cascade type-II infrared InAs/GaSb superlattice photodetectors. Such photodetectors are made up of multiple stages, which are connected in series using an interband tunneling heterostructure. Each stage can be divided into three regions: absorber region, relaxation region and interband tunneling region. Cascade configurations allows to achieve fast response detectors. Making the assumption of bulk-like absorbers, we show how the standard semiconductor transport and recombination equations can be extended to the case of multiplestage devices. We report on the dependence of the Johnson-noise limited detectivity on the absorber thickness for a different number of stages. This allows optimization of the detector architecture, necessary to achieve high value of the detectivity. For this purpose, we make comparison of collection efficiency in single- and multiple-stage devices. The collection efficiency rapidly increases with increasing the number of stages in multiple-absorber detector, especially in situation where the absorber material’s diffusion length is less than absorption depth. We show that the optimal value of the detectivity for different number of stages does not change significantly. The potential benefits of the cascade architecture are shown to be higher in long-term detection regime.
In this work we investigate the high-operating temperature performance of InAsSb/AlSb heterostructure detectors with cut-off wavelengths near 5 μm at 230 K. The devices have been fabricated with different type of the absorbing layer: nominally undoped absorber, and both n- and p-type doped. The results show that the device performance strongly depends on absorber layer doping. Generally, p-type absorber provides higher values of current responsivity than n-type absorber, but at the same time also higher values of dark current. The device with nominally undoped absorbing layer shows moderate values of both current responsivity and dark current. Resulting detectivities D° of non-immersed devices varies from 2×109 to 7×109 cmHz1/2/W at 230 K, which is easily achievable with a two stage thermoelectric cooler.
Interband cascade type-II mid-wavelength infrared (MWIR) InAs/GaSb superlattice (T2SL) detector in room temperature (300 K) is investigated. A single stage in the cascade is a double heterostructure with the absorber sandwiched between electron and hole barriers. The absorber region is non-intentionally doped and is made of MWIR 9ML InAs/9ML GaSb T2SL. At low temperatures structure has a residual doping of p-type, but at room temperature the SL is n-type with n(300K) = 1016cm-3. In this article we calculate dark current being treated as a sum of two currents: average bulk current and average leakage current, flowing through the device. Average bulk current is theoretically calculated, while the average leakage current results from a comparison of theoretically estimated bulk current and measured one. We show that it is possible to fit theoretical model to experimental data, assuming that transport in absorber region is determined by the dynamics of the intrinsic carriers. Based on the fit we estimated carrier lifetime equal about 100 ns at temperature 300 K.
The paper reports on the long-wave (λc = 8.05−11 μm) HgCdTe (Cd composition, xCd = 0.17−0.2) infrared detector for ultra short response time operating for unbiased condition and room temperature (300 K). The optimal structure in terms of the short response time versus device architecture was shown. The response time of the long-wave (xCd = 0.17−0.2) HgCdTe detector for 300 K was calculated at the level of τs ~ 400−440 ns for zero bias condition and lack of the extra series resistance. It was presented that extra series resistance related to the processing (in the range ~ 0−20 Ω) extends response time within the range τs ~ 650−800 ps for active layer xCd = 0.2.
Most of the HgCdTe infrared detectors are fabricated by mesa geometry using a wet chemical or plasma etching techniques. The mesa definition etch process induces undesirable changes in HgCdTe surface properties. In narrow bandgap materials these surface changes could deteriorate a device performance. Uncontrolled band bending occurred on the slopes of the active layer increase of the recombination velocity causing surface leakage current which is a serious problem that affects infrared detectors. Adequate passivation is essential to minimize the effects from the surface states by saturating them. The HgCdTe barrier detectors were investigated for unpassivated and passivated devices. For the unpassivated structure the experimental value of Jbulk (at -0.2 V bias and a temperature of 200 K) was found at the level of 52% of the total dark current for devices with large diameters (500 μm). In the case of detectors with small diameters, the dark current is dominated by the surface leakage current. For a detector with a diameter of 200 μm, the bulk current consists only 28% of the total dark current. After passivation the level of bulk current increase to 58% in cause of 200 μm diameter and almost 75% in cause of large diameter.
In this paper interband cascade type-II InAs/GaSb superlattice photodetector in temperature range from 225 K to 300 K
is investigated. The article concerns the theoretical simulations of the detectivity characteristics of cascade detector with
equal absorber regions in each stage. The obtained theoretical characteristics are comparable to experimentally
measured, assuming that transport in absorber is determined by dynamics of intrinsic carriers. The greatest fit is observed
for overlap values which increase with decreasing temperature form 0.175 eV for 225 K to 0.132 eV for 300 K.
Theoretical and experimental investigations on the response time improvement of unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T=230 K were presented. Metal–organic chemical vapor deposition technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition and donor/acceptor doping and without postgrown ex-situ annealing. The time constant is lower in biased detectors due to Auger-suppression phenomena and reduction of diffusion capacitance related to a wider depletion region. The relatively high bias current requirements and excessive low-frequency noise, which reduces the detectivity of biased detectors, inspire research on the time constant improvement of unbiased detectors. The response time of high-operating temperature LWIR HgCdTe detectors revealed complex behavior being dependent on the applied reverse bias, the operating temperature, the absorber thickness and doping, the series resistance, and the electrical area of the devices. The response time of 2 ns was achieved for unbiased 30×30 μm HgCdTe structures with λ50%=10.6 μm operating at T=230 K.
Theoretical and experimental investigations on the response time improvement of biased and unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T = 230K were presented in this paper. MOCVD technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping and without post grown ex-situ annealing. Donor doping efficiency in (111) and (100) oriented HgCdTe layers has been discussed. The time constant is lower in biased detectors due to Auger suppression phenomena and reduction of diffusion capacitance related to wider depletion region. The relatively high bias currents requirements and excessive low frequency noise which reduces the detectivity of biased detectors inspire researches on the time constant improvement of unbiased detectors. The response time of high-operating temperature (HOT) LWIR HgCdTe detectors revealed complex behavior being dependent on the applied the reverse bias, the operating temperature, the absorber thickness and doping, the series resistance and the electrical area of the devices.
We present progress in metal organic chemical vapor deposition (MOCVD) growth of (100) HgCdTe epilayers achieved recently at the Institute of Applied Physics, Military University of Technology and Vigo System S.A. It is shown that MOCVD technology is an excellent tool for the fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping, and without post grown ex-situ annealing. Surface morphology, residual background concentration, and acceptor doping efficiency are compared in (111) and (100) oriented HgCdTe epilayers. At elevated temperatures, the carrier lifetime in measured p-type photoresistors is determined by Auger 7 process with about one order of magnitude difference between theoretical and experimental values. Particular progress has been achieved in the growth of (100) HgCdTe epilayers for medium wavelength infrared photoconductors operated in high-operating temperature conditions.
Experimental results concerned morphology improvement of HgCdTe layers grown by MOCVD on GaAs substrates are
presented. Selected growth parameters on morphology state have been discussed. The substrate issues like its quality and
crystallographic orientation as well as misorientation play considerable role in final layer smoothness. We study HgCdTe
layer thickness on its surface roughness. The MBE/MOCVD combination method had been adopted for CdTe buffer
layer deposition.
Extensive characterization studies using accessible equipment and methods: atomic force microscopy (AFM), secondary
electron microscopy (SEM), laser scatterometer and Nomarski microscopy have provided invaluable information about
the connection between defect formation and the influence of specific growth parameters.
The performance of multi-layer hetrojunction (MLHJ) HgCdTe photodiodes at high temperatures is presented. The effect of inherent and excess current mechanisms on quantum efficiency and R0A product is analyzed. The diodes with good R0A operability and high quantum efficiency at 200-300 K have been demonstrated at cutoff wavelengths up to 5 μm. The temperature dependence of the differential resistance is discussed. The experimental results show that proper surface passivation and low series/contact resistance are major issues relating to fabrication of HgCdTe detectors with high performance.
This paper describes and assesses growth of Hg1-xCdxTe (MCT) layers by liquid phase epitaxy (LPE) from Te-rich solutions by the tipping (rotating) method. Epitaxial layers with different compositions from x = 0.18 to x = 0.22 and thicknesses from 10 to 20 μm were grown on (111)B oriented CdZnTe substrates. Growth was carried out in the temperature range 460 - 480°C with cooling rates 0.05 - 0.1°C/min and under flowing H2. The attention was paid mainly to the surface morphological quality, good decantation from the layers, uniformity of composition and thickness of films. HgCdTe layers were characterized using different methods: microscopic examinations, infrared microscopic transmission, secondary ion mass spectrometry and scanning electron microscopic measurements. By optimizing the growth parameters and construction of graphite boat it was possible to obtain high quality Hg1-xCdxTe photodiodes.
The performance of very long wavelength infrared (VLWIR) HgCdTe photodiodes at temperatures ranging from 77 K up to 150 K is presented. The effect of inherent and excess current mechanisms on quantum efficiency and dynamic resistance-area RA product is analysed. Different methods of determining the ideality factor are shown and among them the one based on the use of RA product versus bias voltage proves to be the most reliable. At higher temperatures, however, the calculated ideality factor does not give any useful information about the nature of the p-n junction current due to significant influence of the series and shunt resistances. A comparison of the experimental data with the results of analytical and numerical calculations shows that the photodiodes with cut-off wavelength up to 14.5 μm are diffusion-limited at temperatures exceeding 100 K.
This study describes fabrication of heterojunction HgCdTe photodiodes passivated with a wide band-gap CdTe epitaxial layer. The current-voltage characteristics of these photodiodes with and without passivation have been investigated. It is shown that for reverse bias the measured I-V characteristics can be explained by a surface tunneling current and surface generation current. The breakdown voltage is observed to decrease monotonically with increasing temperature, a trend that is directly opposite to what would be expected from a pure tunneling mechanism. Additional information on surface limitations is obtained from analyzing the R0A product as a function of temperature. The performance of both type of p-n VLWIR HgCdTe photodiodes (with and without the passivating layer) have been compared.
In the paper the performance of P-on-n double-layer heterojunction HgCdTe photodiodes are temperature 77 K is analyzed theoretically. Calculation has been performed for the backside-illuminated configuration. The effect of photodiode base layer geometry on quantum efficiency and R0A product is analyzed. The effect of lateral collection of diffusion current and photocurrent on photodiode parameters is also shown. Moreover the dependence of the p-n junction position within heterostructure on the band-gap energy profiles and photodiode performance is presented. Finally, the influence of the composition gradient and p- side doping concentration on photodiode parameters is described briefly.
The material used throughout this study was Mercury Cadmium Telluride (MCT) grown by liquid phase epitaxy (LPE). Due to its special physical and electrical properties, MCT is still one of the most important infrared materials. That's why we are looking for the ways of improving technology and processing and then extracting the best of this material. This article concerns the influence of p-type MCT layer on junction formation during ion etching. To achieve device quality p-type layers, a lot of experiments were performed with annealing of as-grown wafers or adding different quantity of As to the melt. The technological problems with activation of arsenic and with ion etching are shown. Adjusting of parameters of annealing and etching processes allows n-on-p junctions to be formed with a controllable electrical profile. Standard techniques were used to determine optical and electrical parameters of layers.
This paper concerns HgCdTe heterostructure photodiodes for detection of infrared radiation from 8-12micrometers spectral range. Heterostructures were made by liquid phase epitaxy on the CdZnTe substrates. The cap layer of wider energy gap was used to suppress the generation-recombination current from the top contact. P-on-n junctions were placed in the narrow band-gap area close to the graded region. It was accomplished by appropriate selection of the cap layer thickness and adequate choice of parameters of the As diffusion process. Photodiodes were mesa delineated and illuminated through the substrate. Analysis of the impact of p-n junction location within a heterostructure, contact locations and mesa depth on photo diode parameters, was carried out. The experimental results were compared with two-dimensional numerical calculations performed in APSYS.
The successful fabrication of long wavelength Hg1-yCdyTe/Hg1-xCdxTe heterostructures (Y$GTRx) on semi- insulating (111)CdZnTe substrates is presented. The heterostructures consist of a thin 2-5 micrometers layer on n-type 10- 15 micrometers thick HgCdTe epilayer. A novel tipping boat for liquid phase epitaxial growth of mercury cadmium telluride from Te-rich solutions has been proposed. The characterization of double- layer heterostructures was carried out using different methods: microscopic examinations, infrared microscopic transmission, and scanning electron microscopic measurements. Electrical properties were measured in temperature range of 77-300 K using the Van der Pauw arrangement. By optimizing the growth parameters and the construction of graphite boat it was possible to obtain high quality, relatively abrupt Hg1-xCdxTe heterostructures.
This book, originally published December 5th, 2000, has been republished as an eBook October 21st, 2022.
In this monograph, investigations of the performance of narrow-gap semiconductor photodiodes are presented, and recent progress in different IR photodiode technologies is discussed: HgCdTe photodiodes, InSb photodiodes, alternatives to HgCdTe III-V and II-VI ternary alloy photodiodes, lead chalcogenide photodiodes, and a new class of photodiodes based on two-dimensional solids. Investigations of the performance of photodiodes operated in different spectral regions are presented.
A novel tipping boat for liquid phase epitaxial growth of mercury cadmium telluride from Te-rich solutions has been proposed. By optimizing the growth parameters and the construction of graphite boat it was possible to obtain in situ Hg1-xCdxTe heterostructures. The successful fabrication of long wavelength Hg1-yCdyTe/Hg1-xCdxTe heterostructures on semi-insulating CdZnTe substrates is presented. The heterostructures consist of a thin 1-2 micrometers layer on n-type 10-15 micrometers thick HgCdTe epilayer. The characterization of double-layer heterostructures was carried out using different methods. Variations in the layer thickness were determined by microscopic examination of cleaved samples. Average composition was determined from an IR absorption measurement on the central area of the layers. Chemical analysis and the Cd, Hg and Te profile compositions at different depths of the epitaxial layers were performed using secondary ion mass spectrometry. Transport properties were measured in temperature range of 77-300 K using the Van der Pauw arrangement. The paper also reports an experimental innovation of the in situ preparation of the mercury cadmium telluride heterostructures using the tipping method from Te- rich solutions performed in a one-zone reactor.
The RF magnetron sputtering growth and characteristics of ZnS and SiO2 passivants on the surface of bulk n-type InAsSb have been reported. Our investigations include InAsSb surface preparation and in-situ pretreatment, deposition- induced surface damage, interface charge, and thermal stability. The metal-insulator-semiconductor test structures are processed and their electrical properties are measured by capacitance-voltage characteristics. The effect of sputtering growth conditions parameters (deposition temperature, pressure of argon, and RF power) is also reported. The sputtered ZnS layers exhibit excellent dielectric, insulating and mechano-chemical, as well as interface properties. The interfaces characterized by slight accumulation and small (after preservation of the thin natural oxide) hystereses are demonstrated. The SiO2 layers have poor adherence and high density of pinholes. The samples with larger surface state densities have been observed.
Gate-controlled diodes were made by using evaporated indium electrodes overlapping the edge of mesa diodes, isolated from the surface by a layer of ZnS or by native anodic oxide of InSb or HgCdTe. The resulting 3D device characteristics with gate voltage as a parameter have been investigated. Relative spectral responses and I-V characteristics were measured at 77 K. The R0A product is used as an indicator of the dark current of photodiodes passivated with ZnS layer. A plot of R0A values versus gate potential shows that the optimum R0A values are obtained at small positive gate bias voltage. This dependence is consistent with surface recombination influencing the R0A product. The results of a 2D model for calculating gate-induce surface leakage currents due to band-to-band tunneling are presented. The exact quantitative comparison cannot be made between our results and theory, since the active tunneling area is not known.
The growth of In(Bi,Sb) epilayers using liquid phase epitaxy has been discussed. The layers were grown on a (111)B side of InSb substrate using Bi-rich solution in horizontal slider type boat. The growth conditions for high-quality planar epitaxial layers were determined. Mirrorlike surface morphology was observed using a Nomarski differential interference contrast microscope. Hall and resistivity measurements performed at 300 K and 77 K showed an impurity contamination of the epitaxial layers. A capacitance-voltage technique has been established to determine the distribution of doping levels on the surface of InBiSb epilayers. The results indicate that the epitaxial layers of In(Sb,Bi) are n-type at room temperature, however, the time of baking solutions (before crystallization) determined type of conductivity and the concentration of free carriers in epilayers, at 77 K. For short-time-baked solution (from 5 to 20 hours), samples were p-type (carrier concentration approximately 3(DOT)1015 cm-3) when for long- time-baked solutions (40 - 100 hours), samples were n-type (carrier concentration approximately 5(DOT)1015 cm-3). We have observed that type of conductivity depends on surface morphology of the epilayers. The type of doping and the segregation coefficient k for tin for different solutions were established. For In rich solutions tin was an acceptor with k equals 0.0012 and for Bi-rich ones tin was a donor with k equals 0.0039 at 400 degree(s)C.
The surface passivation is essential for the fabrication of high performance HgCdTe photodiodes, especially for photodiodes with small junction area. The fabrication of the HgCdTe photodiodes passivated with a wide band gap epitaxial layer has been described. The planar double-layer heterojunction (DLHJ) structures used in fabrication Hg1- xCdxTe photodiodes were grown on CdZnTe substrates by liquid phase epitaxy (LPE). The P+-n long wavelength infrared radiation (LWIR) photodiodes were fabricated by arsenic diffusion into n-type HgCdTe DLHJ structures. To improve the photodiode performance a thickness of n-type base layer was limited. The photodiodes performances were determined from measurements of the current-voltage and spectral response characteristics. The generation- recombination current was found to be dominant current around zero bias voltage at 77 K. The diodes without antireflection coating had a typical quantum efficiency of 60 percent. The performance of both type of p-n LWIR HgCdTe photodiodes (with and without the wide band gap epitaxial layer) have been compared.
The quality of Hg1-xCdxTe epitaxial layers made of is strongly dependent on the crystalline properties of the substrate. The chemical compatibility and the small lattice mismatch between Hg1-xCdxTe and CdTe have been primary motivation for choosing CdTe as a substrate for Hg1-xCdxTe epitaxial layers. Nevertheless, the most important issue of epitaxial layers on the CdTe substrate is conditioned by lattice mismatch. In order to eliminate these problems we have replaced the CdTe substrate by Cd0.95Zn0.05Te which are lattice-matched to the Hg1-xCdxTe compound. In this work we conduct systematic experimental study of the two type of substrates: (111)B Cd0.95Zn0.05Te and (211)B Cd0.95Zn0.05Te. The (111)-oriented substrate remains attractive as the growth results in flat, featureless surfaces with excellent interfaces with lattice matched substrate. The (211)-oriented substrates combine the structural quality of (100) including the absence of twinning with the flat topography of (111)-oriented wafers. The Hg1-xCdxTe epilayers were grown from Te-rich solutions on (111)Cd0.95Zn0.05Te and (211)Cd0.95Zn0.05Te by a horizontal tipping liquid phase epitaxy technique. Characterization of the epilayers involved FTIR spectroscopy to determine both its thickness and composition. LPE film surface morphology was examined using microscope equipped with Nomarski phase contrast and atom force microscopy. The as- grown or annealed layers were measured by Hall effect at 300 and 77 K using Au or indium contacts.
Andrzej Mycielski, A. Szadkowski, E. Lusakowska, Leszek Kowalczyk, J. Domagala, Jadwiga Bak-Misiuk, B. Witkowska, Krzysztof Adamiec, Jaroslaw Rutkowski, A. Jedrzejczak
The growth of the single crystals of ZnTe and Cd1-xZnxTe (x <EQ 0.25) by a simple, horizontal, low temperature physical vapor transport technique (PVT) is presented as an example of the application of the PVT technique to the technology of MBE substrates: large (1 inch in diameter), twin-free crystals of wide-gap II-VI compounds of very high quality. The advantages of the PVT technique are emphasized. The results of the characterization of the grown crystals are presented.
In this study, we report the RF magnetron sputtering growth and characterization of CdTe passivant on bulk n-type HgCdTe. Our investigations include the HgCdTe surface preparation and in-situ pretreatment, deposition-induced surface damage, interface charge, CdTe film stoichiometry, and thermal stability. The metal-insulator-semiconductor test structures are processed and their electrical properties are measured by capacitance-voltage characteristics. The heterostructures are also characterized by reflectance measurement. In order to investigate the passivation properties of CdTe/HgCdTe heterostructures, we have modeled the band diagram of abrupt CdTe/HgCdTe heterojunction. The effect of sputtering growth condition parameters is also reported. The sputtering CdTe layers, exhibit excellent dielectric, insulating and mechano- chemical properties, as well as interface properties. The interfaces are characterized by slight accumulation and a small hysteresis. A carefully controlled growth process and surface pretreatment tailored to the specific material are required in order to obtain near flat band conditions on n- type materials. Additional informations on surface limitations are obtained from analyzing the I-V characteristics of photodiodes with metal gates covering the p-n junction surface location.
Results of a study of the optical and electrical performance of P-on-n HgCdTe photodiode configuration are presented. The effect of doping profile and heterostructure contacts with wide gap region on the photodiode parameters is solved by forward-condition steady-state analysis. An isotype heterojunction, a high/low junction or very heavily doped high quality p-type material is required to achieve competitive performance from this device architecture in the diffusion limited temperature regime. It is shown that the isotype heterojunctions have the advantages over a high/low homojunctions for IR detectors. The wide gap region has low thermal generation rates of carriers and it also isolates the active region of the device from carrier generation at the contacts. Results obtained from computer simulations were found to be in qualitative agreement with previous experimental data on HgCdTe photodiodes.
In this paper, the growth of p-n InAsSb/InSb heterojunction using liquid phase epitaxy (LPE) has been discussed. The layers have been grown on B InSb substrate using an In-rich solution in horizontal slider type boat. The active InAsSb layer was first grown with a desired composition. The carrier concentrations in the top layer in the range 1016 cm-3 to 1020 cm-3 was easily controlled using Cd doping. The standard structure consisted of 100 micrometers heavily doped n-type InSb substrate, an 10 micrometers InAsSb active region, and 2 micrometers heavily doped InSb p-type cap layer. Mirror like surface morphology was observed using a Nomarski differential interference contrast microscope. The structural characterization and the composition of InAsSb have been determined from x-ray diffraction data and IR transmission characteristic. The technology and construction of mesa photodiodes, both backside and frontside illuminated, have been presented. The analyses of the R0A product and current-voltage characteristics as a function of temperature shows that the dark currents of InSb/InSb photodiodes are diffusion limited. At higher As composition the R0A product is affected by the generation-recombination current of the depletion region.
The fabrication of HgCdTe IR detectors demands high-quality CdTe or CdZnTe substrates. Bulk CdTe tends to twin, therefore large single crystals are generally not available. This problem could be circumvented by growing CdTe epilayers on an alternative large area substrate. Several studies have been made on the growth of CdTe on different substrates such as InSb, GaAs, Si and sapphire by MOCVD and MBE techniques. We report the initial results for the growth of CdTe buffer films on GaAs (100) substrates by sputter epitaxy. This crystal was chosen as the substrate material because of its transparency to IR radiation and availability as large area wafers with high structural perfection. Epitaxial films of CdTe were deposited in a sputtering system with a base pressure of 2 X 10-4 Pa. The GaAs substrate was degreased, etched in standard solution, and mounted immediately on a cooper substrate holder in the system. The substrates were ion etched in the sputtering system to remove surface oxide. The CdTe films were deposited in a wide substrate temperature range from 50 to 450 degrees C. Film thickness ranged from 0.1 to 5 micrometers , and deposition rates from 1 to 5 micrometers /h. The orientations and crystalline quality epitaxial films were characterized by x-ray diffraction. The surface morphology and the cross section of the gown CdTe layers were investigated by Nomarski interference contrast microscope. The optical and the electrical properties of the epitaxial films were investigated too. Structural characterization reveals that crystalline quality is a function of temperature of substrates. The single-crystals films grown at 300 degrees C on GaAs showed a best surface morphology.
Medium- and long-wavelength infrared (MWIR and LWIR) Hg1-xCdxTe photodiodes (x equals 0.265 - 0.295, 0.205 - 0.220) for 3 - 5-micrometer and 8 - 12- micrometer wavelength spectral regions were fabricated by arsenic diffusion from the vapor source into the n-type HgCdTe bulk single crystals. The temperature dependence of current- voltage and capacity-voltage characteristics, and the photodiode speed response, were analyzed. For 3 - 5-micrometer photodiodes with the electron concentration 3 multiplied by 1015 cm-3 in the base n-type layer, the typical resistance-area product at zero bias RoA was about 104 (Omega) cm2 at 77 K. For 8 - 12 micrometer photodiodes with the same electron concentrations in the base region, the RoA product values of 1 - 10 (Omega) cm2 were obtained. The typical photoresponse speeds were in the range of 50 ns and 20 - 30 ns for 3 - 5-micrometer and 8 - 12-micrometer photodiodes, respectively. The dark current of fabricated photodiodes was tunneling limited at 77 K, and generation-recombination and diffusion limited at higher temperatures.
P+-n long wavelength infrared radiation (LWIR) photodiodes were fabricated by arsenic diffusion into n-type HgCdTe bulk monocrystals. To improve the photodiode performance, a thickness of n-type base layer was limited. The photodiodes performance was determined from measurements of the current-voltage and spectral response characteristics. The generation-recombination current was found to be a dominant current around zero bias voltage at 77 K.
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